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4. The Medium Access Sub Layer
Broadcast channels [or multi-access channels] are a categoryof networks and the key
issue is how todeterminewhogets to use the channelwhen there is competition for it. The
protocols which define these factors belong to a sub layer of data link layer called the
MAC(medium accesscontrol)sub layer.
ALOHA: Norman Abramson devised a new and elegant method to solve the channel
allocation problem calledthe ALOHA system which usedground-based Radio broadcasting.
Twocategoriesare presentinthis ALOHA system. They are:
a. Requiresglobaltimesynchronization [SLOTTED ALOHA]b. Doesntrequireglobaltimesynchronization. [PURE ALOHA]
(a). SLOTTED ALOHA: Roberts publisheda methodfordoublingthecapacityofan ALOHA
system by dividing the time up into discrete intervals, with each interval corresponding to a
singleframe. Timesynchronizationwasachieved byhavingaspecialstationthatemitsa pip at
the start of each interval, like a clock. In this system, a computer is not permitted to send
wheneveracarriagereturnistyped. Instead, itisrequiredtowaitforthe beginningofnextslot.
Sincethevulnerability period isnowhalved, the probabilityofnoothertrafficduringthesome
slotise-gleadsto.
S = Ge-g
Throughput Vsofferedtrafficgraph:
From thegraph, SLOTTED ALOHA peaksat G=1 withathroughputofabout 0.368.
If system is operating at G=1, Probability of empty slot is 0.368 i.e. 36% of channel is
utilized.
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OperatingathighervaluesofG
y Reducesthenoofempties.y Increasesthenoofcollisionsexponentially.
(b).PURE ALOHA: Inan ALOHA system, userstransmitwhenevertheyhavedatato besent
infixedlengthframes.whencollisionoccur, thesendergetstheinformationduetothefeedback
propertyofbroadcasting. Iftheframewasdestroyed, thesender justwaitsarandom amountof
timeandsendsitagain.
Frame generation in an ALOHA system:
User
A
B
C
D
E
If the first bitofanew frameoverlapswith just the last bitofa framealmost finished,
bothframeswill betotallydestroyedand bothwillhaveto bere-transmittedlater.
Letthe meanframe (new)generated bydifferentnumberofusers perframetime be s(frames
withoutcollisions). ThevalueofS can betherefore, either0 or1.
FRAME TIME:theamountoftimeneededtotransmitthestandard, fixedlengthframe
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(1)
Letthe meanframes (new + retransmitted)generated bydifferentnoofusers perframetime
withoutcollisions be G. ThevalueofG isobviouslygreaterthanofequalto S(2)
Atlowloadi.e., S=0
Therewill befewcollisions.so, fewretransmissionsarerequired. So,
(3)
Athighloadi.e., S=1.
Therewill be manycollisions.so, fewre-transmissionsarerequired. So,(4)
IfP0isthe probabilitythataframedoesntsufferfrom anycollision,
(5)
IfPr[k]isthe probabilitythat kframesaregeneratedduringagivenframetime, then
(6)
The probabilityof0framesis Pr[0] = G0e-G
K!
P0 = e-G
( 7)
The meanno.offramesgeneratedinanintervalof2 frametimeslongisgiven by 2G.
The probabilityofnoothertraffic beinginitiatedduringtheentirevulnerable periodisgiven by
P = e-2G
(8)
The maximum throughputoccursat G=0.5with S=1/2ei.e., S=0.184 i.e.,
0S
S=GP0
Pr[k] = Gk
e-G
K!
With Pure ALOHA, 18% channel utilization is made
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Carrier Sense Multiple Access Protocols:
Thereare3typesofcarriersense protocols. Theyare:
1. 1-Persistant CSMA2. Non-Persistent CSMA3. p-Persistent CSMA
(1)1-Persistant CSMA: Whenastationhasdatatosend, itfirstlistenstothechanneltoseeif
anyone else is transmitting at that moment. If the channel is busy, the station waits until it
becomesidle. Whenthestationdetectsanidlechannel, ittransmitsaframe. Ifacollisionoccurs,the station waits a random amount of time and starts all over again. The protocol is called
1-persistant becausethestationtransmitswitha probabilityof1 wheneveritfinds channelidle.
Problems:-
1. Ifastation becomesreadytosend (justafteranotherstation beginssending),ifsensesthechannelto beidle(becauseofpropagationdelayofthefirst)andwill beginsending, which
resultsinacollision.
2. If2 stations becomereadyinthe middleofthirdstationstransmission, bothwill politelywait untilthetransmissionendsand bothwill begintransmittingexactlysimultaneously,
resultinginacollision.
2.Non-persistent CSMA:- A station senses the channel before sending. IF no one else is
sending, thestation beginsdoingso itself. If thechannel isalready in use, thestationdoesnot
continually sense it for the purpose of seizing it immediately upon detecting the end of the
previoustransmission. Insteaditwaitsarandom periodoftimeandthenrepeatsthealgorithm.
Def:- Protocols in which stations listen for a carrier (i.e. transmission) and actaccordingly are called Carrier Sense Protocols.
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3.p-Persistent CSMA:- Itappliestoslottedchannels. Whenastation becomesreadytosend, if
senses thechannel. If it is idle, it transmitswitha probability Pwitha probabilityq=1-p it
differs until the next slot. If that slot is also idle, it either transmits or defers again, with
probabilities p and q. This process is repeated until either the frame has been transmitted or
anotherstationhas began transmitting. If thestation initially senses thechannel busy, itwaits
untilthenextslotandappliestheabovealgorithm.
II.(b). CSMA with collision Detection:
In this protocol, the stations abort their transmissions as soon as they
detect a collision .If 2 stations sense the channel to be idle and begin transmitting
simultaneously, they will both detect the collision almost immediately. Rater than finish
transmitting their frames, which are irretrievably garbled anyway, they should abruptly stop
transmittingassoonasthecollisionisdetected. Quicklyterminatingdamagedframessavestime
and bandwidth. This protocol is known asCSMA/CD and is usedwidely usedon LANs in
MAC sub layer.
Conceptual model ofCSMA/CD:-
to
Transmission Contention Contention idlePeriod Period slots Period
Time
Frame Frame Frame Frame
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Atthe point marked to, astationhasfinishedtransmitting itsframe. Anyotherstation
havingaframetosend maynowattempttodoso. Asactualtransmissionsandretransmissions
(ifanycollisionoccurs)occur, ourmodelforCSMA/CD willconsistofalternatingtransmission
andcontention periods, withidle periodsoccurringwhenallstationsarequit.
If2 stations begintransmitting bothexactlyattime to, theyrealizethattherehas beena
collision bydeterminingthelengthofcontention periodandthusthedelayandthroughput.
The minimum time todetect thiscollision is the time taken by the signal to propagate
from onestationtotheother.
IEEE standard 802 for LANs:
Severalstandards produced by IEEE whichinclude CSMA/CD token busandtokenringare
collectivelyknownas IEEE 802.
802.1 standardgivesanintroductiontothesetofstandardsanddefines I/fprimitives. 802.2 standardgivesthe upperpartofdatalinklayerwhich uses LLC protocol. 802.3standarddescribesthe LAN standard.CSMA/CD 802.4standarddescribesthe LAN standard Token bus. 802.5standarddescribesthe LAN standardTokenring.
IEEE 802.3: The 2.94mbps CSMA/CD system built by Xerox PARC connects over 100
personalstationsona 1 k.m cablecalled Ethernet, inwhich thesystem uses ALOHA along
withcarriersensingtechnique. Later, Xerox, DEC and INTEL drew up astandardfora 10-mbps
Ethernet, called the 802.3,which describes a whole family of 1-PERSISTENT CSMA/CD
systemsrunningatspeedsfrom 1 to 10mbpsonvarious media.
a).cabling: 4typesofcablingarecommonly usedin802.3
1. 10 base-52. 10 base-23. 10 base-T4. 10 base-F
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1) 10 base-5: Thisis popularlycalled ThickEthernet. Itresemblesayellowgardenhouse,
with markingsevery 2.5mts to showwhere the tapsgo. Connections to itaregenerally made
using Vampire Taps, inwhicha pin iscarefully forcedhalfway into thecoaxialcablescore.
The notation 10base5 means that it operates at 10 mbps, uses base band signaling and can
supportsegmentsofup to500mts.
Transceiver: Itisclampedsecurelyaroundthecablesothatitstap makescontactwiththeinner
core. It contains the electronics that handle carrier detection and collision detection. When a
collisionisdetected, it putsaspecialinvalidsignalonthecabletoensurethatalso putsaspecial
invalidsignal, on thecable toensure thatallother transceiversalsorealize thatacollisionhas
occurred.
Transceiver Cable: Itconnectsthetransceivertoan I/fboardinthecomputer. It may be upto
50mtslongandcontains5individuallyshieldedtwisted pairs:
y 2 pairsfordatain & dataout.y 2 pairsforcontrolsignalsin & out.y 1 pairto powertransceiverelectronics.
I/F board: Itcontainsacontrollerchip thattransmitsframesto, receivesframesfrom
transceiver.
Controller: Itisresponsibleforassemblingthedatainto properframeformat , aswellas
computingchecksumsonoutgoingframesandverifyingthem onincomingframes.
Note: The lengthofthen/wcan beextended bythe useofrepeaters betweenany 2 stations.
Thisstandardallowsa maximum of4repeaters in the path betweenany 2 stations, extending
theeffectivelengthofthe medium to 2.5km.
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2) 10 base-2: This is popularly called Thin Ethernet. Connections to it are made using
industrystandardBNC connectorstoform T-junctions. Itruns foronly 200mtsandcanhandle
30 machines percablesegment. It is muchcheaperandeasier to install. Theconnection tothe
cable is just a passive B NC T-junction connector. The transceiver electronics are on the
controllerboardandeachstationalwayshasitsowntransceiver.
3) 10 base-T: Thisdefinesastar-shapedtopologyinwhichallstationshaveacablerunningtoa
centralhub. Usuallythesewiresaretelephonecompanytwisted pairs. Inthis, thehub actsasthe
repeaterinwhichitrepeatsthesignalontheoutgoinglinetoeachstation, whenasinglestation
transmits.
Time Domain Reflectometry: Todetectcable breaks, badtapsorlooseconnectors, a pulse
ofknownshapeisinjectedintothecable, bywhichanechowill begeneratedandsent back,
If it hitsanobstacleorendof cable.Bycarefully timing the interval between sending the
pulseandreceivingtheecho , originofechocan belocalized.
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Advantages:
y With 10base-T thereisnocableatall, justthehub.y Addingorremovingastationissimplerinthisconfigurationy Cable breakscan bedetectedeasily.y Maintenanceiseasy.
Disadvantages:
y The maximum cablerunfrom thehub isonly 100mtsy Largehub coststhousandsofdollars.
4) 10 base-F: This uses Fiberoptics, whichhasexcellentnoiseimmunityanditisthechoiceof
methodwhenrunning between buildingsrwidelyseparatedhubs.
Switched 802.3 LANs:
As moreand morestationsareaddedto 802.3 LAN, thetrafficwillgo up andeventually,
the LAN will saturate. The solution is togo tohigher speed, say from 10mbps to 100mbps,
whichisachievedthrougha switched802.3 LAN.
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Theheartofthissystem isaswitchcontainingahigh-speed backplaneandroom fortypically
4to32 plug-inlinecards, eachcontaining 1 to98connectors. Mostoften, eachconnectorhasa
10 base-T twisted pairconnectiontoasinglehostcomputer.
Whenastationwants totransmit802.3frames, itcomputesastandard frame totheswitch
the plug-in cardgetting the frame checks to see if it isdestined forone of the other stations
connected to thesamecard. Ifso, the frame iscopied there. Ifnot, the frame is sentover the
high-speed backplanetypicallyrunsatover1 gap usinga proprietary protocol.
Problem: Do 2 machinesattachedtothesame plug-incardtransmitframeatsametime?
Solution1: Form alocalonward LAN withallthe plotsonthecard, wiringtogether. So,
collisionswill bedetectedandhandledasinan CSMA/CD n/w, with
retransmission using backoffalgorithm.
Solution2: Each I/P portis backoffalgorithm soincomingframesarestoredinthecardson-
board RAM astheyarrive. Onceaframehas beencompletelyreceived, thecard
canthenchecktoseeiftheframeisdestined post, andtransmitaccordingly. As
each portisaseparatecollisiondomain, nocollisionsoccur.
IEEE standard 802.4: Token Bus
Physically Token bus is a linear or Tree-shaped cable onto which the stations are attached.
Logically the stations are praised into a ring with each station knowing the address of the
stationtoits left and right. Whenthelogicalringisinitialized, thehighestnumberedstation
may send the first frame. After it isdone, it passes permission to its immediate neighbor by
sending the neighbor a special control frame called a token which propagates around the
logical ring, with only the token holder being permitted to transmit frames. Since only one
stationata timeholds the token, collisionsdonotoccur. The token bus uses 75- broadband
coaxialcableforphysicallayerwhich permits3differentanalog modulationschemes:
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1. Phasecontinuous Frequency Shift Keying.2. Phasecoherent Frequency Shift Keying3. Multilevelduo binaryamplitude modulated Phase Shift Keying.
The Token Bus MAC Sub layer protocol:
When thering is initializedstationsare inserted into it inorderofstationaddress, from
highest to lowest. Token passing isalsodone from high to lowaddresses. Each timeastation
acquires the token; itcan transmit frames foracertainamountof timeand then, it passes the
tokenon. Ifastationhasnodata, it passesthetokenimmediately upperreceivingit.
Thetoken busdefines4 priorityclasses 0, 2, 4and6fortraffic, with 0 the lowestand6thehighest. When the tokencomes into the stationover thecable, it is passed initially to the
priority6 substation, which may begin transmitting frames, if ithasany. When it isdone, the
tokenis passedinternallyto priority4substationwhich maythentransmitframes untilitstimer
expires atwhich point the token is passed internally to priority 2 substation. This process is
repeated untileitherthe priority 0 substationhassentallofitsframesoritstimerhasexpired. At
this point , thetokenissenttothenextstationinthering.
Logical Ring Maintenance:
Addition of new station into the Ring: Once the ring has been established, each stations
interface maintainstheaddressofthe predecessorandsuccessorstationsinternally. Periodically,
the tokenholdersendsof the SOLICIT-SUCCESSOR frames tosolicit bids from stations that
wishto jointhering. Theframegivesthesendersaddressandthesuccessorsaddress. Stations
inside thatrange may bid toenter. Ifnostation bids toenterwith inaslot time, theresponse-
windowisclosedandthetokenholdercontinueswithitsnormal business. Ifexactlyonestation
bidstoenter, it is insertedintotheringand becomesthetokenholderssuccessor. If2 ormore
stations bid to enter, their frameswill collide and be garbled. The tokenholder then runs an
arbitrationalgorithm, startingwith the broadcastofa RESOLVE-CONTENTION frame. Each
stationhasatimerthatisresetwheneveritacquiresthetoken.
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Leaving of a station from the ring: A station Xwith successor S and predecessor P
leaves the ring, by sending P a SET-SUCCESSOR frame telling it that henceforth its
successorsis SinsteadofX. Then X justtransmitting.
Initialization of a Ring:
o Itisaspecialcaseofaddingnewstations. Assoonasthesystem is powered ON andifitnoticesthatthereisnotrafficforacertain period, itsendsa CLAM - TOKEN Frame.
o Itcreatesatokenandsets up aringcontainingonlyitself.o Periodicity, itsolicits bidsfornewstationsto join.o Asnewstationsare ON, theywillrespondtothese bidsand join bid usingcontentionalg.
Problems with Logical Ring / Token due to Transmission Errors:
1. If a stations tries to the token to a station that has gone down..Sol: Afterpassingthetoken, astationlistenstoseeifitssuccessoreithertransmitsaframe
orpassesthetokenandifsothetokenis passedasecondtime. Ifthatalsofails, thestation
transmitsaWHO-FOLLOWS framespecifying theaddressof its successorseesa WHO-
FOLLOWS frame naming its predecessor, it responds by sending a SET-SUCCESSOR
frametothestationwhosesuccessorfailed, named itselfasthenewsuccessor. Inthisway,
thefailedstationisremovedfrom thering.
2. If a station fails to pass the token to its successor and also fails to locate thesuccessor and also fails to locate the successors successor, which may also be down.
Sol: The system sends a SOLICITSUCCESSOR2 frame to see if anyone else is still
alien. Onceagainthestandardcontention protocolisrun, withallstationsthatwantto bein
theringnow biddingfora place. Eventuallytheringisreestablished
3. If the token holder goes down and takes the token with it.Sol: Thisissolved using Ring Initializationalgorithm. Eachstationissuesahitsathreshold
value, thestationissuesaCLAIMTOKENframeandcontentionalgorithm determineswho
getsthetoken.
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4. Multiple TokensSol:
y If a station holding the token notices a transmission from another station, itdiscardsitstoken.
y Iftherewere 2, therewouldnow beone.y Iftherewere morethan 2, this processwould berepeatedsoonerorlateruntilall
butonewerediscards.
y If by accident, all tokens are discarded, then lackof activitywill causeone ormorestationstotrytoclaim token.
3. IEEE Standard 802.5 : Token Ring
A token ring isnotreally a broadcast medium, butacollectionof individual point-to-
point links thathappens to form acircle, whichcanrunon twisted pair, coaxialcableor fiber
optics. A majorissueinthedesignandanalysisofanyringnetworkisthe physicallength ofa
bit. If thedatarateof thering is R mbps, thena bit isemittedevery 1/R secwitha typical
signal propagationspeedofabout 200m/ sec, each bitoccupies 200/R mtsonthering.
Eg:In a 1 mbps ring with circumference of 1000mts, the length of each bit = 200/1000=1/5.
i.e., only 5 bits are present on the ring, at once.
A tokenringconsistsofacollectionofringinterfacesconnected by point-to-pointlines. Each bit
arriving atan interface iscopied intoa 1-bit bufferand thencopiedouton to the ringagain.
Whileinthe buffer, the bitcan beinspectedand possibly modified before beingwrittenout. This
copying bitintroducesa 1-bitdelayateachinterface.
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Ina tokenring, aspecial bit patterncalled the token, circulatesaroundtheringwhen
everall stations are idle. Whena stationwants to transmita frame, it is required to seize the
token byinvertingasingle bitin3-bytetoken.
Logically it isa ring but physically, eachstation isconnected to the wirecenter bya
cablecontaining 2 twisted pairs (at least)one fordata tothestationandone fordata from the
station. Inside thewirecenterare bypassrelays thatareenergized bycurrentfrom thestations.
Thering breaksorastationgoesdown, lossofdrivecurrentwillreleasestherelayand bypass
thestation. Theringcanthencontinueoperationwiththe badsegment bypassed
The Token Ring MAC
sub layer protocol :
When there is no traffic or the ring, a 3-bits token circulates endlessly waiting for a
stationtosizeit bysettingaspecific 0 bittoa 1 bit, thusconvertingthetokenintothestart-
of-framesequence. Thestationthenoutputstherestofanormaldataframe.
1 1 1 2 or6 2 or6 nolimit 4 1 1
SD AC FC Destinationaddress Sourceaddress DATA CheckSum ED FS
Frame Control Ending Delimiter
Access Control Frame Status
Starting Delimiter
A station mayholdthetokenfortheToken-Holdingtime.
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SD: startingdelimitermarksthe beginningoftheframe
ED:startingdelimitermarkstheendingoftheframe
AC:containsthetoken bits, monitorbits, priority bitsandreservation bits
FC:distinguishesdataframesfrom various possiblecontrolframes
DA: destinationaddress
SA: sourceaddress
CheckSum: checksum field.
FS:itcontains Aand C bits.
o Whenaframearrivesattheg/fofastationwiththedestinationaddressthei/fturnson A bitasit passesthough.
o Ifthei/fcopiestheframetothestation, itturnson C bit.A C Meaning
0 0 Destinationnot presentornot powered up
1 0 Destination presentframenotaccepted
1 1 Destination presentandframecopied
Ring Maintenance: Thetokenring protocolhandles maintenance bythe presenceofmonitor
station that theoverseas ring. When the ringcomes up oranystationnotices that these isno
monitor, itcan transmitaC
LAIM TOKENcontrol framenavigates the ring beforeanyotherCLAIM TOKEN framesaresent, thesenderbecomesthenew monitor.
Monitors Responsibilities:
1. Toseethatthetokenisnotlost2. Totakeactionwhenthering breaks3. Toclearthering up whengarbledframesappear.4. Towatchoutfororphanframes
Orphan frame:
Itisaframethatoccurswhenastationtransmitsashortframeinitsentiretyontoalongring
andthencrashesoris powereddown beforetheframecan bedrained. Ifnothingisdone, the
framewillcirculateforever
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The Token Ring Control Frames:
1. To checkfor lost token: The monitorhasatimerthatissettothelongest possibletokenless
Interval.eachstationtransmittingforthefulltoken-holdingtime. Ifthistimergoesoff, the monitor
drainstheringandissuesanewtoken.
2.When a garbled frame appears: The monitorcandetectit byitsinvalidformatorchecksum,
opentheringtodrainit, andissueanewtokenwhentheringhas beencleaned up.
3. When orphan frame is detected:Anorphanframeisdetected bysettingthemonitorbitin
Access Control byte whenever it passes through. If an incoming frame has this bit set,
something iswrong since the same frame has passed the monitor twicewithout having been
drained, sothe monitordrainsit.
4. When the Ring breaks:A stationtransmitsaBEACONframe, [ifitnoticesthateitherofits
neighborsappears to bedead]giving theaddressof presumablydead station, bywhich itcan
knowthenoofstationsdownanddeletethem from thering usingthe bypassrelaysinthewire
center.
Bytes 1 1 1
Staring Delimiter
Access Control
Ending Delimiter
Control field Name Meaning
00000000 Duplicateaddresstext Textif2 stationshavesameaddress
00000010 Beacon Usedtolocate breaksinthering
00000011 Chaintoken Attemptto become Monitor
00000100 Purge Reinitializethering
00000101 Active monitorpresent Issued periodically bythe monitor
00000110 Standby monitorpresent Announcesthe presenceofpotential monitors
SD AC ED
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A tokenringconsistsofacollectionofring interfacesconnected by point-to-point lines. Each
bitarrivingataninterfaceiscopiedintoa 1-bit bufferandthencopiedoutontotheringagain.
While in the buffer, the bitcan be inspectedand possibly modified before beingwrittenout.
Thiscopying bit introducesa 1-bitdelayateach interface. Inatokenring, aspecial bit pattern
called the token, circulates around the ringwhen ever all stations are idle. When a station
wants to transmita frame, it is required to seize the token by invertinga single bit in3-byte
token.Because there isonlyone token, onlyone station can transmit at a given instant, thus
solvingthechannelaccess problem. Thetokenring mustitselfhavesufficientdelaywhichhas 2
components:
1. 1-bitdelayintroduced byeachstation2. Signal propagationdelay.
A ringinterfacehas 2 operating modes:
(a). Listen Mode: Inthis mode, thei/p bitsaresimplycopiedtoo/p withadelayof1-bittime.
(b). Transmit Mode: This modeisenteredonlyafterthetokenhas beenseized, thei/fbreaks
theconnection betweeni/p ando/p enteringitsowndataintothering.
Problem with a ring network: Ifthecable breakssomewhere, theringdies.
Solution: WIRE CENTER .
4 stations connected via a Wire Center:
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BRIDGES
Def: Thedevices bywhich multiple LANscan beconnectedarecalledBRIDGES and
thesedevicesoperatein Data LinkLayer.
Multiple LANS connected bya backbonetohandleatotalloadhigherthanthecapacityofsingle LAN:
Reasons to construct Bridges:
802.3 802.4 802.51. The protocolissimple. 1. The protocoliscomplex. 1. The protocoliscomplex.
2. Delayatlowlocaliszero. 2.Substantialdelayatlow
Load.
2. Substantialdelayatlowload.
3. Substantialanalogcomponent. 3. Noanalogcomponent. 3. Noanalogcomponent.
4. Prioritiesarenot possible. 4. Prioritiescan beassigned. 4. Prioritiescan beassigned.
5. Asthespeedincreasesthe
Efficiencydecreases.
5. Asthespeedincreasesthe
Efficiencyincreases.
5. Asthespeedincreasesthe
Efficiencyincreases.
6. Athighload, the presenceofcollision seriouslyaffectthe
Throughput.
6. Athighload, ithasexcellentThroughput & efficiency.
6. Athighload, ithasexcellentThroughput & efficiency.
7. It usesa passivecable. 7. It useshighlyreliablecableTelevisionequipment.
7. Ringsone built usingvirtuallyanytransmission medium from
Carrierpigeontofiberoptics.Standardtwisted pairischeap.
8. Automaticdetection &
eliminationofcablefailuresisnot possible.
8. Automaticdetection &
eliminationofcablefailuresisnot possible.
8.Automaticdetection &
eliminationofcablefailureisdone bythe useofwirecenters.
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1. Many universitiesandcorporatedepartmentshavetheirown LANsinwhichgoalsofvarious
departmentsdiffer.Bridgesareneededwhenthereisneedforinteraction betweenthese
variousdepartments.
2. Theorganization may begeographicallyspreadoverseveral buildingsseparated by
considerabledistances. It may becheapertohaveseparate LANsineach buildingandconnect
them with bridgesratherthantorunasinglecoaxialcableovertheentiresite.
3. It may benecessarytosplitwhatislogicallyasingle LAN intoseparate LANsto
accommodatetheload.
4. Insomesituations, asingle LAN would beadequateintermsoftheload, butthe physical
distance betweendistant machinesistoogreat.(Eg: >2.5kmsfor802.3). Even iflayingthe
cableiseasytodo, thenetworkwouldnotworkduetotheexcessivelylonground-trip delay.
Theonlysolutionisto partitionthe LAN andinstall bridges betweenthesegments.
5.Bridgescancontributetoorganizationssecurity. Most LAN interfaceshavea promiscuous
mode, inwhichallframesaregiventothecomputer, not justthoseaddressedtoit.By
inserting bridgesatvarious placesand beingcarefulnottoforwardsensitivetraffic, itis
possibletoisolate partsofthenetworksothatitstrafficcannotescapeandfallintowrong
hands.
6. Thereisno matterofreliability. Onasingle LAN, adefectivenodethat keepsoutputtinga
continuousstream ofgarbagewillcripplethe LAN.Bridgescan beinsertedatcritical places
to preventasinglenodewhichhasgone berserkfrom bringingdowntheentiresystem.
Problems:
1. Eachofthe LANs usesadifferentframeformat. Anycopying betweendifferent LANsrequirereformatting, whichtakes CPU time, requiresanewchecksum calculationand
introducesthe possibilityofundetectederrorsdueto bad bitsinthe bridges memory.
2. Interconnected LANsdonotnecessarilyrunatthesamedatarate. Whenforwardingalongrunofback-to-backframesfrom afast LAN toaslowerone, the bridgewillnot be
abletogetridoftheframesasfastastheycomein. Itllhaveto bufferthem, hopingnotto
runoutofmemory.
3. Thevalueoftimersinhigherlayersisa bottleneckproblem in bridges. Supposethatthen/wlayerinan802.4 LAN istryingtosendaverylong messageasasequenceofframes.
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Aftersendingthelastone, itstartsitstimertowaitforanacknowledgement. Ifthe
message hastotransmita bridgetoaslower802.5 LAN, thereisadangerthatthetimer
willgooffbeforethelastframehas beenforwardedontotheslowerLAN. Then/wlayer
willassumethatthe problem isduetoalostframeand justretransmitstheentiresequence
again. Afternfailedattempts, it maygive-up andtellthetransportlayerthatthe
destinationisdead.
4. Allthethree LANshaveadifferent maximum framelength. For802.3,802.4,802.5standards, the payloadis 1500, 8191 and5000 bytesrespectively.
5. Anobvious problem iswhenalongframe must beforwardedontoa LAN thatcannotacceptit, whichhasnosolution. Framesthataretoolargeto beforwarded must be
discarded.
Bridges from 802.x to 802.y(problems):
: Theonlythingthatcangowrongisthatthedestination LAN isso
heavilyloadedthatframeskeep pouringintothe bridge, butthe
bridgecannotgetridofthem. Ifthissituation persistslongenough,
the bridge mightrunoutofbufferspaceand begindroppingframes.
: Wehavethe problem ofwhatto putinthe priority bits.
: The bridge mustgenerate priority bits.
: (a). 802.4framescarry priority bitsthat802.3framesdonothave.
(b). Temporarytokenhandofffeatureof802.4
: Theonly problem iswhattodowiththetemporarytokenhandoff.
: (a). Potential problem withframesthataretoolong.
(b). Temporarytokenhandoffproblem.
: The802.5frameformathas A and C bitsintheframestatus byte.
These bitsareset bythedestinationtotellthesenderwhetherthe
stationaddressedsawtheframeandwhetheritcopiedit. Here, a
bridgecanlieandsaytheframehas beencopied, butifitlater
turnsoutthatthedestinationisdown, serious problems mayarise.
2.3 2.4
2.3 2.5
2.4 2.3
2.4 2.4
802.4 802.5
2.5 2.3
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: (a). Definitionofpriority bitsisdifferentforthe 2 LANs
(b). The A and C bitsofframestatus bytein802.5
: Whattodowith A and C bitsisthe majorproblem.
DifferentkindsofBridges:
1. TransparentBridges.2. Spanning TreeBridges.3. Source RoutingBridges.4. RemoteBridges.
1. Transparent Bridges:
Thisisthefirst802 bridge, whichoperatesin promiscuous modeacceptingeveryframe
transmittedonall LANstowhichitisattached.
Eg: A configurationwith4 LANsand 2 Bridges:
Bridge B1 connected to LANs 1 and 2.
Bridge B2 connected to LANs 2, 3 and 4.
A framearrivingat bridgeB1 on LAN1 destined for Acan bediscarded immediately,
because it isalreadyon theright LAN, buta framearrivingon LAN1 for Cand F must be
forwarded. Whenaframearrives, a bridge mustdecidewhethertodiscardorforwardit, andif
the latter , on which the LAN to put the frame. This decision is made by looking up the
destination address in a big (hash) table inside the bridge. The table can list each possible
destinationandtellwhichoutputline (LAN)it belongson. Whenthe bridgesarefirst pluggedin,
all thehash tablesareempty. Noneof the bridgesknowwhereanyof thedestinationsare, so
they usefloodingalgorithm.
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The algorithm used by the transparent bridges is BACKWARD LEARNING. Itworks as
follows:
y The bridgesoperatein promiscuous modeandtheyseeeveryframesentonanyoftheirLANs.By lookingat the sourceaddress, theycan tellwhich machine isaccessibleon
which LAN.
Eg: Intheabovefigure, Ifbridge B1seesaframeon LAN2 comingfrom C, itknows
that C must be reachable LAN2, so it makes an entry in its hash table noting that
framesgoingto Cshould use LAN2. Anysubsequentframeaddressedto Ccomingon
LAN1 will beforward, butaframeforCcomingon LAN2 will bediscarded.
y Thetopologycanchangeas machinesand bridgesare powered up anddownand movedaround. Tohandledynamictopologies, wheneverahashtableentry is made, thearrival
timeoftheframeisnotedintheentry. Wheneveraframewhosedestinationisalreadyin
thetablearrives, itsentryis updatedwiththecurrenttime. Thus, thetimeassociatedwith
everyentrytellsthelasttimeaframefrom that machinewasseen.
y Periodically, a processinthe bridgescansthehashtableand purgesallentries morethanafew minutesold. Inthisway, ifacomputeris unpluggedfrom its LAN, movedaround
the buildingandre-pluggedinsomewhereelse, withinafew minutes, itwill be backin
normaloperation, withoutany manualintervention.
Routing: The Routing procedure foran incoming framedependson the LAN itarriveson(thesource LAN)andthe LAN itsdestinationison (the Destination LAN), asfollows:
1) Ifthedestinationandsource LANS arethesame, discardtheframe.
2) Ifthedestinationandsource LANS aredifferent, forwardtheframe.
3) Ifthedestination LAN is unknown, useflooding.
Aseachframearrives, thisalgorithm must beapplied.
Flooding Algorithm:
Everyincomingframeforan unknowndestinationisoutputonallthe LANstowhichthe bridge
isconnectedexcepttheone itarrivedon. Astimegoeson, the bridges learnwheredestinations
are. Onceadestination isknown, framesdestinedforitare putononlythe properLAN andare
notflooded.
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The unknown Destination is handled using flooding i.e., copies it to LAN2 in the above
example. Shortly there after, bridge1 sees F2, a framewith an unknown destination, which it
copies to LAN1, generating F3. Similarly, bridge2 copies F1 to LAN1 generating F4 .Bridge1
nowforwards F4and bridge2 copies F3. Thiscyclegoesonforever.
2. Spanning Tree Bridges :
As paralleltransparent bridgescreateloopsinthetopology, spanningtree bridgesare
employed. Inthis, the bridgescommunicatewitheachotherandoverlaytheactualtopologywith
aspanningtreethatreachesevery LAN .Ineffect, some potentialconnections between LANS are
ignoredintheinterestofconstructingafictitiousloop-freetopology.
In the above example, 9 LANS are interconnected by 10 bridges. This configuration can be
abstracted into a graphwith the LANS as the nodes. An arc connects any 2 LANS that are
connected bya bridge. Thegraphcan bereducedtoaspanningtree bydroppingthearcsshown
asdottedlines.
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A spanning tree covering the LANS :
Using this spanning tree, there is exactlyone path from every LAN to every other
LAN .Once the bridges have agreed on the spanning tree , all forwarding between LANS
follows the spanning tree. Since there isa unique path from eachsourcetoeach destination,
loopsare impossible.
Building a spanning tree (using DISTRIBUTED ALGORITHM):
y First, the bridgeshavetochooseone bridgeto betherootofthetree. Each broadcastisassigned its uniqueserialnumberandthe bridgewith lowestserialnumber becomesthe
root.
y Next, atreeofshortest pathsfrom theroottoevery bridgeand LAN isconstructed. Thistreeisthespanningtree.
y Ifa bridgeorLAN fails, anewoneiscomputed.Theresultofthisalgorithm isthata unique path isestablishedfrom every LAN totherootand
thustoeveryotherLAN.
3. Source Routing Bridges:
This scheme is employed by token ring people where as Transparent bridges are
employed by TokenBus & CSMA/CD. The Source Routing assumes that the sender of each
frame knowswhether or not the destination is on its own LAN. When sending a frame to a
different LAN, thesource machinegetshigh -order bitof thesourceaddress to 1, to mark it.
Furthermore, itincludesinthe Frameheadertheexact paththattheframewillfollow.
This pathcan beconstructedasfollows:--
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1. Each LAN hasa unique 12-bitnumberandeach bridgehasa4-bitnumberthat uniquelyidentifies it in the context of its LANS. A route is then a sequence of bridge, LAN,
bridge, LAN,numbers.
2. A source routing bridge is only interested in those frames with high-order bit ofdestinationsetto 1.
3. Foreachsuchframethatitsees, itscanstheroutelookingforthenumberofthe LAN onwhichtheframearrived.
4. Ifthis LAN numberisfollowed byitsown bridgenumber, the bridgeforwardstheframeontothe LAN whosenumberfollowsits bridgenumberintheroute.
5. If the number of some other bridge follows the incoming LAN number, it does notforwardtheframe.
This Algorithm leads itself to 3 possible implementations:
(a). Software: The bridgerunsin promiscuous mode, copyingallframestoits memorytoseeif
theyhavethehigh-orderdestination bitsetto 1. Ifso, theframeisinspectedfurther. Otherwise,
itisnot.
(b). Hybrid: The bridges LAN interfaceinspectsthehigh-orderdestination bitandonlyaccepts
frameswith the bit set. This If is easy to build into H/W andgreatly reduces the number of
framesthe bridge mustinspect.
(c). Hardware: The bridges LAN interfacenotonlychecksthehigh-orderdestination bit, butit
alsoscanstheroutetosee ifthis bridge mustdoforwarding. Onlyframesthat mustactually be
forwarded are given to the bridge. This implementation requires the most complex H/W but
wastesno bridge CPU cycles becauseallirrelevantframesarescreenedout.
In source Routing, every machine in the internet, knows or can find, the best path to
everyother machine. These routersarediscovered from the basic idea that Ifadestination is
unknown, thesourceissuesa broadcastframeaskingwhereitis. TheDISCOVERY FRAMEis
forwarded byevery bridgeso that it reachesevery LAN or the internetwork. When the reply
comes back, the bridgesrecord their identity in it, sothattheoriginalsendercansee theexact
routetakenand ultimatelychoosethe bestroute.
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DISADVANTAGE: Itsuffersfrom aframeexplosion. ConsideraseriesofLANS connected by
tripleBridges:
1. Each discovery frame sent by station 1 is copied by eachof the3 bridgeson LAN1,yielding3discoveryframeson LAN2.
2. Eachofthese is copied by each ofthe bridgeson LAN2, resultingin9 frameson LAN3.
3. Bythetimewereach LAN N, 3N-1
framesarecirculating.
4. Ifadozensetsofbridgesaretraversed , morethana milliondiscoveryframeswillhaveto beinjectedintothelast LAN, causingseverecongestion.
Comparison of IEEE 802 bridges:
S.no Issue Transparent Bridge Source Routing Bridge
1 Orientation Connectionless Connection orientation
2 Transparency Fullytransparent Nottransparent
3 configuration Automatic Manual
4 Routing Sub optimal Optimal
5 Locating Backward Learning Discovery frames
6 Failures Handled by bridges Handled byhosts
7 Complexity Inthe bridges Inthehosts
8 Compatibility Compatible Not Compatible
9 Parallel bridges Notapplicable Applied b/w 2 LANS tosplitthe Load
10 N/W Management Notneeded N/Wmanagerinstalls LAN & Bridgenumbers
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4. Remote Bridges :
Used toconnect 2 ormoredistant LANS. Forexample , acompany mighthave plantsinseveral
cities, eachwithitsown LAN. Ideally ,allthe LAND should be interconnectedsothecomplete
system acts likeone large LAN. Thisgoalcan beachieved by puttinga bridgesoneach LAN
andconnectingthe bridges pairwisewith point-to-pointlines.
Example :considerasystem with3 LANS:
The3 point-to-pointlinesareregardedashostless LANS . Thenwehaveanormalsystem of6
LANS interconnected by4Bridges.
Various protocols can be used for Point-to-Point lines:
y Tochoosesomestandard Data link protocol, puttingcomplete MAC frames in payloadfield.
y Tostrip offthe MAC headerandtraileratsource bridgeand putwhat is left in payloadfieldofpoint-to-point protocol. A new MAC headerandtrailercanthen begeneratedat
thedestination bridge.